Ignition engines that use very lean air-fuel mixtures have been developed, that is, having a higher air composition to reduce fuel consumption and emissions. In order to provide a safe ignition it is necessary to have a high energy ignition source. Prior art systems generally use large, high energy, single spark ignition coils, which have a limited spark duration and energy output. To overcome this limitation and also to reduce the size of the ignition system multi-charge ignition systems have been developed. Multi-charge systems produce a fast sequence of individual sparks, so that the output is a long quasi-continuous spark. Multi-charge ignition methods have the disadvantage that the spark is interrupted during the recharge periods, which has negative effects, particularly noticeable when high turbulences are present in the combustion chamber. For example this can lead to misfire, resulting in higher fuel consumption and higher emissions.
An improved multi-charge system is described in European Patent EP2325476 which discloses a multi-charge ignition system without these negative effects and, at least partly, producing a continuous ignition spark over a wide area of burn voltage, delivering an adjustable energy to the spark plug and providing with a burning time of the ignition fire that can be chosen freely.
However there are still various problems with such systems. It is not possible to control the secondary current, which results in a high spark plug wear as well as a large amount of wasted energy which is not required for combustion. Furthermore at the end of the ignition cycle a high secondary current peak can be generated, which results in a high spark plug wear.
Furthermore in such systems, the PWM-signal of the step-down-converter stage is adapted to a fixed value, which results in a non-stable primary current under various conditions.
Aspects of the invention are provided as stated in the claims.